This invention relates to a communication apparatus and method of format adaptation therefor, and in particular for use in a CDMA cellular communication system for mobile communication.
In a cellular communication system each of the subscriber units (typically mobile stations) communicates with typically a fixed base station. Communication from the subscriber unit to the base station is known as uplink and communication from the base station to the subscriber unit is known as downlink. The total coverage area of the system is divided into a number of separate cells, each predominantly covered by a single base station. The cells are typically geographically distinct with an overlapping coverage area with neighbouring cells. FIG. 1 illustrates a cellular communication system 100. In the system, a base station 101 communicates with a number of subscriber units 103 over radio channels 105. In the cellular system, the base station 101 covers users within a certain geographical area 107, whereas other geographical areas 109, 111 are covered by other base stations 113, 115.
As a subscriber unit moves from the coverage area of one cell to the coverage area of another cell, the communication link will change from being between the subscriber unit and the base station of the first cell, to being between the subscriber unit and the base station of the second cell. This is known as a handover. Specifically, some cells may lie completely within the coverage of other larger cells.
All base stations are interconnected by a fixed network. This fixed network comprises communication lines, switches, interfaces to other communication networks and various controllers required for operating the network. A call from a subscriber unit is routed through the fixed network to the destination specific for this call. If the call is between two subscriber units of the same communication system the call will be routed through the fixed network to the base station of the cell in which the other subscriber unit currently is. A connection is thus established between the two serving cells through the fixed network. Alternatively, if the call is between a subscriber unit and a telephone connected to the Public Switched Telephone Network (PSTN) the call is routed from the serving base station to the interface between the cellular mobile communication system and the PSTN. It is then routed from the interface to the telephone by the PSTN.
A cellular mobile communication system is allocated a frequency spectrum for the radio communication between the subscriber units and the base stations. This spectrum must be shared between all subscriber units simultaneously using the system.
One method of sharing this spectrum is by a technique known as Code Division Multiple Access (CDMA). In a Direct Sequence CDMA (DS-CDMA) communication system, the signals are prior to being transmitted multiplied by a high rate code whereby the signal is spread over a larger frequency spectrum. A narrowband signal is thus spread and transmitted as a wideband signal. At the receiver the original narrowband signal is regenerated by multiplication of the received signal with the same code. A signal spread by use of a different code will at the receiver not be de-spread but will remain a wide band signal. In the receiver the majority of interference caused by interfering signals received in the same frequency spectrum as the wanted signal can thus be removed by filtering. Consequently a plurality of subscriber units can be accommodated in the same wideband spectrum by allocating different codes for different subscriber units. Codes are chosen to minimise the interference caused between subscriber units typically by choosing orthogonal codes when possible. A further description of CDMA communication systems can be found in xe2x80x98Spread Spectrum CDMA Systems for Wireless Communicationsxe2x80x99, Glisic and Vucetic, Artech house Publishers, 1997, ISBN 0-89006-858-5. Examples of CDMA cellular communication systems are IS 95 standardised in North America and the Universal Mobile Telecommunication System (UMTS) currently under standardisation in Europe.
Future cellular communication systems such as UMTS will support many different applications and services. In order to provide these services efficiently, a number of different transmission formats will be supported. Examples of different transmission formats includes different spreading rates, modulation formats, forward error correcting codes and interleaving formats. The receiver must take the current transmission format into account and in UMTS, it is intended that information on the transmission format will be embedded in the communication. Specifically, it is intended that control bits (known as TFCI bits) will be included to signal the transmission format to the receiving units. On a block by block (frame by frame basis) the transmitter sets the TFCI bits to indicate the format used to encode the user data. The received TFCI is used in the receiver to perform the appropriate decoding of the user data. The types of parameter, which may be affected by the TFCI include: the spreading factor or modulation, the channel coding, rate matching, interleaving and service multiplexing. In UMTS the TCFI bits together with pilot and other information is transmitted on one quadrature channel while the user data is transmitted simultaneously on the other quadrature channel.
However, current schemes have a number of disadvantages. One of these is that using the TFCI bits to indicate the transmission format in the subsequent frame causes an inherent delay which for UMTS is 10 msec, which is unacceptable for some services. However, using the TFCI bits to indicate the transmission format in the current frame requires that the TFCI bits are demodulated and decoded before the demodulation of the user data can commence. This can result in a significant increase in the complexity of the receiver as the raw signal samples must be stored. For example, in UMTS the TCFI bits are spread throughout a frame and the decoding of these bits cannot be completed until the end part of the frame. As a result a receiver must store a whole frame of signal samples at least at the chip rate rather than storing symbol samples. As the spreading factor can be up to 256 times or higher, this results in significantly increased storage requirements.
The inventors of the current invention have realised that conventional approaches for performing transmission format adaptation has a number of disadvantages including having inherent delays and/or requiring very complex receivers. The invention seeks to provide advantages including reducing delays for a number of transmission format adaptations while not requiring significant increase in the complexity of receivers.
Accordingly there is provided a communication apparatus comprising receiving means for receiving transmissions from a transmitter, the transmissions comprising transmission format information; and adjustment means for adjusting at least one characteristic of the communication apparatus in response to the received transmission format information;
wherein the at least one characteristic is adjusted for the transmission block in which the transmission format information was received when the received transmission format information belongs to a first subset of transmission format information and for at least one subsequent transmission block when the received transmission format information belongs to a second subset of transmission format information.
Preferably the first subset comprise transmission format information relevant to symbol level processing and the second subset comprises transmission format information relevant to chip level processing. Specifically, the first subset may comprise transmission format information chosen from the group of a channel coding; an interleaving format; a transmission rate; and a multiplexing of services; and the second subset may comprise transmission format information chosen from the group of a spreading factor; a spreading modulation; a spreading code; and a modulation format. According to a feature of the invention the communication apparatus is used in a cellular CDMA communication system.
According to a different feature of the invention, the transmission format information is communicated as transmission format data and a mapping between the transmission format data and a transmission format parameter is dynamically determined.
According to a second aspect of the invention, there is provided a communication system comprising transmission means for transmitting a signal comprising transmission format information; receiving means for receiving the signal transmitted from the transmission means; and adjustment means for adjusting at least one characteristic of the receiving means in response to the received transmission format information wherein the at least one characteristic is adjusted for the transmission block in which the transmission format information was received when the received transmission format information belongs to a first subset of transmission format information, and for at least one subsequent transmission block when the received transmission format information belongs to a second subset of transmission format information.
According to a third aspect of the invention there is provided a method of format adaptation in communication system comprising the steps of: transmitting a signal comprising transmission format information; receiving the signal transmitted from the transmission means; and adjusting at least one characteristic of the receiving means in response to the received transmission format information wherein the at least one characteristic is adjusted for the transmission block in which the transmission format information was received when the received transmission format information belongs to a first subset of transmission format information, and for subsequent transmission blocks when the received transmission format information belongs to a second subset of transmission format information.
The invention thus provides an improvement in the transmission format adaptation and specifically it provides reduced delay for many transmission format adaptations while not requiring significantly increased complexity of the receivers.